re-add missing css class in admin sidebar template

What does this MR do?

Re-add wrapper class from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2717

See CE version: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/layouts/nav/_new_admin_sidebar.html.haml#L2

Are there points in the code the reviewer needs to double check?

Don't believe so

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?